Radiation-use efficiency of a forest exposed to elevated concentrations of atmospheric carbon dioxide

Abstract

We compared radiation-use efficiency of growth (epsilon;), defined as rate of biomass accumulation per unit of absorbed photosynthetically active radiation, of forest plots exposed to ambient (approximately 360 micro l l-1) or elevated (approximately 560 micro l l-1) atmospheric CO2 concentration ([CO2]). Large plots (30-m diameter) in a loblolly pine (Pinus taeda L.) plantation, which contained several hardwood species in the understory, were fumigated with a free-air CO2 enrichment system. Biomass accumulation of the dominant loblolly pines was calculated from monthly measurements of tree growth and site-specific allometric equations. Depending on the species, leaf area index (L*) was estimated by three methods: optical, allometric and litterfall. Based on the relationship between tree height and diameter during the first 3 years of exposure, we conclude that elevated [CO2] did not alter the pattern of aboveground biomass allocation in loblolly pine. There was considerable variation in L* estimates by the different methods; total L* was 18-42% lower when estimated by the optical method compared with estimates from allometric calculations, and this discrepancy was reduced when optical measurements were corrected for the non-random distribution of loblolly pine foliage. The allometric + litterfall approach revealed a seasonal maximum total L* of 6.2-7.1 with about 1/3 of the total from hardwood foliage. Elevated [CO2] had only a slight effect on L* in the first 3 years of this study. Mean epsilon; (+/- SD), calculated for loblolly pine only, was 0.49 +/- 0.05 and 0.62 +/- 0.04 g MJ-1 for trees in the ambient and elevated [CO2] plots, respectively. The 27% increase in epsilon; in response to CO2 enrichment was caused primarily by the stimulation of biomass increment, as there was only a small effect of elevated [CO2] on L* during the initial years of fumigation. Long-term increases in atmospheric [CO2] can increase epsilon; in closed-canopy forests but the absolute magnitude and duration of this increase remain uncertain.

Canopy leaf area constrains [CO 2 ]-induced enhancement of productivity and partitioning among aboveground carbon pools

“…This decrease of Ϸ10% for a near doubling of L may reflect the effect of shoot structure on the light environment within the canopy and a lesser ability of P. taeda to increase SLA. The SLA in the bottom third of the pine was only 30-50% greater than SLA in the top third (39). Moreover, unlike the finding for the broadleaf forests, elevated [CO 2 ] enhanced the fractional allocation to wood at all levels of L D .…”

Leaf dynamics of a deciduous forest canopy: no response to elevated CO 2

“…LAI is more difficult to measure in the evergreen, needle-leaf Pinus taeda stand in the FACE experiment in North Carolina, and reports have been inconsistent regarding the effect of [CO 2 ] (Lichter et al 2000;Luo et al 2001;DeLucia et al 2002). If there have been any effects of [CO 2 ] in that experiment, apparently they have been small.…”

Forest response to elevated CO2is conserved across a broad range of productivity

“…The annual increments of stem wood (I wood ) and coarse woody root (I coarse root ) were estimated by applying site-specific allometric equations to periodic measurements of the diameter of trees in current and elevated CO 2 plots. Harvest data from the POP-EUROFACE (19) and AspenFACE experiments (20); and the tree height-diameter relationship for trees at DukeFACE (21) indicate that exposure to elevated [CO 2 ] did not affect the allometric relationships between plant parts. The allometric equation used at ORNL-FACE incorporated basal area, height, taper, and wood density to reduce the possibility of the allometry being altered by elevated [CO 2 ] (15).…”

“…The combined contribution of herbivory and dissolved organic C in the DukeFACE experiment was only 3% of NPP (18), so it is unlikely that their exclusion would alter our results. Absorbed photosynthetically active radiation (APAR, or the light energy in megajoules absorbed by the canopy) was estimated from the difference between quantum sensor measurements of down-welling radiation (400-700 nm) and the amount reaching the forest f loor (POP-EUROFACE and ORNL-FACE) (19,25) or by applying a Beer's Law approximation to measurements of leaf area index (LAI) (21). Values of APAR were integrated over the growing season.…”